1. Field of the Invention
For a long time, refractometers have been widely used to determine the optical refractive index in liquids and gases in which in the most diverse forms of the device the exit angle of the refracted light ray in its transition from the measurement medium to a reference medium is determined quantitatively. The basis of these measuring instruments is Snellius' Law of Optical Refraction. The achievable accuracy of the refractive index of the medium being studied is thus dependent, among other factors, on the accuracy of the optical refractive index of the reference body and of the angles α and β of the incident and the refracted light ray, as well as on certain characteristics of the light source and of the detector measuring the incoming ray. In order to achieve maximum accuracy in measurements it is necessary to fulfill the highest stability requirements, especially in some optical/mechanical components and their alignment in the optical bank of the measurement instrument. The absolute values of the angles, the measurements and the refractive index of the reference body need not necessarily be known very exactly, since the refractometer can be calibrated using one or more liquids and/or gases the precise optical refractive index of which is/are exactly known.
2. Description of the Related Art
Refractometers of especially high accuracy in in situ measurements in the ocean, as well as in the laboratory, have a significant role in determining the physical state quantities of ocean water, especially in the extensive spaces of the deep sea. Thus for a long time attempts have been made to create appropriate instruments for field use, but the stability and/or accuracy achieved has remained somewhat unsatisfactory. Nonetheless, refractive index accuracy in the range of 10−6, possibly even to 10−7, is required for meaningful refractive index measurement in the ocean, which means that in practice refractive angle measurements which are stable and maximally long-term constant in the magnitude of one-tenth arc second must be achieved at hydrostatic environmental pressures up to ca. 1000 bar.
To date it could be shown that such high stability requirements are basically achievable, as for example evident in recently described and experimentally tested field instruments. (OCEANS '88, IEEE Publ. No. 88-CH 2585–8, Baltimore, Md., USA, Volume 2(4), 497 . . . 504, (1988); OCEANS '99, MTS/IEEE Publication, Seattle, Wash., USA, ISBN: 0-933957-24-6, Vol. 3, 1218 . . . 1222, (1999)).